The nice thing about Flo is they've done the leg work for you. Looks like an old SS is the best but your gp4ks are close.

http://flocycling.com/aero_tire_power.php

To answer your specific question: 25.

Flo designed the wheels around the 23mm GP4Ks and these optimize the aero shape. But subsequent testing revealed that the 25mm tire has only a slight aero penalty, despite the mild mushroom shape, and which should be swamped by meaningful improvement in rolling resistance *at a given tire pressure* for the fatter tire. Hence Flo now recommends 25mm GP4Ks *if* they fit on your bike.

The 25mm tires will also yield a cushier ride, albeit also slightly squishier handling, and will allow lower PSI with less pinch flat risk, though depending on the road surface, lowering PSI may increase rolling resistance. As with all things, there are sweet spots to be found, but invariably some tradeoffs.

niccolo wrote:

...Hence Flo now recommends 25mm GP4Ks *if* they fit on your bike.

Are you sure about that? Their website FAQ still recommends the 23 mm tires. Where did you find that info?

http://www.flocycling.com/faq.php

Thanks!

They have posted here a few times about the advantage of the 25mm, if it will fit. Here are a couple recent posts. If you search on keywords "continental," "25," or "25mm," and the poster of "Canadian," you will find a bunch of them.

http://forum.slowtwitch.com/...ost=6181620#p6181620
http://forum.slowtwitch.com/...ost=6351881#p6351881

However, given more recent testing with normalized casing pressure it's a little less clear. I think either would work, but if you're going upwards of 40kph where aero significantly outweighs rolling resistance my intuition says the 23 might be better.

Anyone know if the new SS tires will perform similarly to the old ones as far as aero goes?

Ah, I see. I guess it boils down to Continental GP 4000's are 23's but are shaped more like 25's when mounted.

Thanks!

Why wouldn't you consider the Conti TT? Not a high mileage tire like the 4000s but still has the Vectran puncture protection.....lighter....faster.
Flo apparently didn't test the tire on their chart but if your looking for maximum speed over mileage, this might be your choice.

I am sure.

They recommend the 25mm GP4K because at equivalent tire pressure, the rolling resistance is meaningfully lower, while the aerodynamic penalty is far smaller. And that's for wheels designed around the 23mm GP4K.

But equivalent pressure = greater casing tension, arguably one should be normalizing for casing tension, not PSI. I suspect that's the direction the industry is moving. Logic would suggest that at equivalent casing tension, the shape of a larger tire's contact patch would still lead to lower rolling resistance, up to a point.

The reason they haven't changed the website, they say, is that some frames won't fit a 25mm GP4K.

Would it make sense to have a 23 up front where the air is cleaner and a 25 rear to take advantage of the low rr (but presumably with little to no aero penalty, assuming it fits your frame)?

In a way it makes little difference to me because my decision has been made - just got a good deal on a new set of GP4Ks ($34/tire!) and got one of each. But still curious if the theory holds water.

Actually, that's what I'm currently running on my Flo 30s on my road bike.

But I think they would argue that the 25s are warranted for both wheels.

It largely depends how fast you go. Drag increases exponentially with speed, so there is a point where you're better off with more rolling resistance but also more aero. I'm also not convinced that 23mm tires don't have similar rolling resistance to 25mm tires when they're both inflated to the proper pressure for their respective width. So tl.dr. yes, there is a case when that is a valid thing to do

Thank you very much!

the 4000s is *never* the way to go

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Wasn't the 23 in the FAQ in relation to their Alu models, IE isn't that still the fastest for that front wheel?

Maurice

Run the 25s on both of mine too (Flo 30). With road debris/hazards, I feel much more comfortable going over something inadvertently on the 25s. Plus road surfaces vary so much that I rather like the slight deflection/road buzz reduction on the 25s.

You're also right about the GP4Ks price and availability.

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Indoor Triathlete - I thought I was right, until I realized I was wrong.

Anyone know which frames won't fit with Flo carbon wheels and 25mm conti 4000s? I'm a little worried my P2 won't take them (I have a 2016 model black P2). Planning on getting some Flos this winter and I'd love to fit 25's on them. Any advice appreciated. Can't find anything on Cervelo's website regarding this (tire clearence).

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Just to add to the decisions, I have done quite nicely using the Flo 60/90/disk carbon and carbon/Al tubeless as well.

Canadian's opine on 25mm tires for a P2: http://forum.slowtwitch.com/...ost=5601263#p5601263

I had S Works Turbo tyres which are 24mm and fit the rims pretty flush.

Punctured a few times early on but pretty good since and I think feel faster than GP4000S Ii